It was also revealed that ABC Electronic debutant the Independent.co.uk posted 6.53m unique users in its first published results.
Mail Online jumped 3.73% from April to May, as the Guardian and Telegraph suffered declines.
The Mail's network of sites, which includes the combination of DailyMail.co.uk and MailonSunday.co.uk, is up 100% year on year. However, just 5.09m or 27% of its total unique users came from the UK in May.
Telegraph.co.uk, which overtook the number one spot from Guardian.co.uk last month, declined 0.79% during May, falling to second place with 18.5m unique users.
However, the site's unique users are still up by a whopping 154% year on year and the Telegraph attracts a higher percentage of UK users -- 6.08m users or 33% logged in from the UK.
Guardian.co.uk has now slipped to third place with 18.32m unique users, which is up 14.08% year on year, but down 1.2% month on month.
Times Online held onto fourth position with a 3.06% increase in May to 15.88m unique users. This figure is up 81.87% on May 2007. Around 5.67m of the Times unique users are from the UK.
The Sun's network of sites, which includes TheSun.co.uk, Page3.com and NewsoftheWorld.co.uk, watched its unique users climb 6.25% to 14.95m in May, which is a 66% increase on May 2007. Just over 34% or 5.13m of The Sun's unique users are from the UK.
Independent.co.uk, which relaunched in January, published its first ABCe figure today, showing it had 6.53m unique users.
It also supplied ABCe-approved figures for December which were not published at the time, showing it had 3.29m unique users.
The number of its UK unique users from the UK in May was 2.72m, which is a rise of 86% from December.
Bill Swanson, managing director of Independent New Media, said: "As you would expect from The Independent, the new site is stylish and pleasant on the eye, but what has been really pleasing has been the response to the improved navigation of the site, the new services offered with our partners, and ultimately the response in visits to the site.
"It won't stop here; we are already putting in place a range of new functions and services on the site to further improve engagement with our audience."